Hot water problems driving me mad!

[Izitso]

Hello, we had a new combi boiler fitted last Summer and have had endless trouble with fluctuating water temperature. The company have been out twice but found nothing they could list as a fault. It seems pointless calling then again as it's our word against theirs. We understand there would be a delay in hot water reaching the kitchen as mains is downstairs and boiler is upstairs so water has a long way to travel. We are trying to live with it until the service is due but hot water for washing dishes either comes on hottish and fades to lukewarm or comes on lukewarm and eventually gets to hottish. In the bathroom we've noticed that, if mixer tap is set to fill the bath, we only need to run the hot tap and temp is perfect without adding cold but if we try the shower setting the water is scalding. The pressure is ferocious through the shower fitting and the whole mixer tap (hot and cold side) is very hot to touch. Can't explain it any better than that but just wondered if anyone has experienced anything similar? Thank you.

Anyone? I'm thinking of trying a new mixer tap before getting a plumber in - thinking it might be some sort of valve problem inside the actual tap. Don't think we'll ever solve the fluctuating water temperature downstairs though.

Going to ask a really daft question here but have you checked what temperature the boiler is aiming to deliver hot water at?

Our combi (Ideal Max Vogue) has dials for hot water and central heating, they set the maximum temperature the boiler aims to get to.

Turn any other hot tap on just enough to trigger the combi.
Does the water get hot ... and stay hot?

If you open up the tap a bit more, does the water stay hot?

Ensure there’s no other hot water or central heating demand on the combi while you do this.

Thanks so much for taking the trouble to help. Boiler is a Viessmann set at 60 for water. When tap is partially turned on in bathroom temp fluctuates from hot to lukewarm. When fully open water is hot and stays Fahrenheit steady but gauge changes up and down from 56 to 66.

Fairly steady not Fahrenheit steady!

The bath tap - take the side off the bath, it may have been installed with a mixer which prevents kids/elderly scalding themselves so only allows the water to come out of the tap at a certain temp - ours has this but the shower is diverted past it and so runs much hotter.

Might be going off at a tangent , but you say the gauge changes up and down... the only gauge on ours is the water pressure gauge, usually pointing at 1 or slightly higher. When the boiler is on , that rises, pointing closer to 2.
What model Viessman boiler is it OP?

Thank you both for the input. It's a Vitodens 100-W
There is a separate pressure gauge to the left.

Hi, we have the same boiler and exactly the same issue ! British Gas have been out 5 times, changed almost every boiler part but still an issue.

Did you ever get this sorted ? I know the post is fairly old now but any help would be appreciated !!